Pictures speak for themselves.
Try replicate this.
Rhino 7 SR10 2021-8-11 (Rhino 7, 7.10.21223.07001
Can you post a file with that curve?
How did you create the curve?
i dont remember but either directly with polyline or with bunch of trimmed lines joined together, i always run simplifycrv though
Hm - it looks like the tool is paying attention to curve direction. I’ll get this on the pile, thanks for pointing it out. Flip the curve to get a positive value. SimplifyCrv also fixes this one - it is a polycurve and not a polyline, though that should not matter and I cannot reproduce this, so far, from scratch.
-Pascal